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13 CHAPTER TWO SURVEY OF STATEWIDE TRAVEL FORECASTING PRACTICE SURVEY METHODOLOGY simpler techniques, a majority reported using linear trend lines applied to historical count data. A few states use The survey of states involved four stages; taking advantage growth factors and one state (Wisconsin) uses BoxCox re- of a Statewide Travel Demand Models Peer Exchange held gression, which produces a nonlinear trend line, heavily in September 2004. First, the 14 states planning to attend the weighted toward higher traffic volumes. BoxCox regres- Peer Exchange were asked to answer several open-ended sion is described in the Guidebook (Horowitz 1999). South questions about their models, model creation, and model ap- Dakota establishes growth rates by regression analysis of plication. Second, an analysis of these responses was used to business data and historical vehicle-miles traveled (VMT) create the multiple-choice questions that would be answered treads by county (Johnson 2000). A number of states find it by some or all states. Third, a very short screening question- necessary to modify their historical trend lines with local naire was prepared (see Appendix A) and e-mailed to all knowledge and other forecasts of business and population states to ascertain their general level of modeling capability growth. It is not uncommon for a state to develop growth fac- and alternatives to modeling. Fourth, those states found to be tors by highway functional class and by region within the reasonably far along in their model development process state. Commercial vehicles are sometimes forecasted sepa- were mailed one of two follow-up questionnaires. The longer rately from passenger cars. Wyoming reported using a mov- of the two follow-up questionnaires was sent to states not ing-average linear regression technique. None of the states participating in the Peer Exchange and the shorter form was reported using BoxJenkins (sometimes called ARIMA or sent to those states represented at the Peer Exchange. The autoregressive integrated moving average) methods, which longer form is found in Appendix B. The shorter form omit- have become essential tools of business forecasting. (For a ted questions that appeared to be adequately covered by the more complete discussion of BoxJenkins methods see the Peer Exchange questionnaire. Guidebook.) All states except Hawaii responded to the screening ques- Other approaches incorporate modeling concepts but stop tionnaire. Of all the states with models, only Louisiana and short of a full-blown statewide model. Kansas reported using Oregon did not return the follow-up questionnaire; however, OD table estimation from ground counts as a stopgap before both states gave extensive responses to the Peer Exchange developing their own statewide model. New York encourages questionnaire and provided model documentation. its MPOs to extend their models into rural areas to achieve a wider coverage and authorizes special models to be built Montana's response to the survey indicated that it did not when needed. Other states reported reliance on MPO models have a statewide model; however, its HEAT (Highway Eco- where possible. nomic Analysis Tool) (Cambridge Systematics, Inc.; Eco- nomic Development Research Group; ICF Consulting; and South Dakota has pursued an interesting variation on Short Elliott Hendrickson, Inc. 2004) has an embedded trendtrend line forecasting that seems to embody principles freight component that is similar in structure to those in other of behavioral travel forecasting. statewide models. Responses from Montana included here were based on a report about HEAT. STATES WITH STATEWIDE MODELING CAPABILITY RURAL TRAFFIC FORECASTING NOT INVOLVING STATEWIDE MODELS Unlike MPO models, which are often permanent components of the UTP process and get incremental upgrades, statewide All states without models and some states with models han- models go through a life cycle. Many of the statewide models dle project-level traffic forecasts through simpler techniques, are in transition; they are either being developed or redevel- such as growth factors and trend lines. Many states do not oped from scratch or are being extensively revised. Other have a fixed methodology that applies to projects in general, models are dormant and one state is considering the possibil- whereas other states have implemented a standard technique ity of building a model. Table 1 gives an overview of the sta- for use everywhere. Of the 32 states reporting that they used tus, at the time of data collection for this synthesis, of all states'